It is often necessary to display both graphics and video on a screen such as a cathode ray tube (CRT). This is especially true when the display is employed as part of a graphics workstation.
Prior art devices for displaying multiple graphic processes provided a fixed, that is, not changeable (without redrawing), combinations of text and graphics on a screen. Once the manner of presentation was chosen, it always appeared in that way. For example, if it was determined that text should always overlay the graphics then it would always appear that way. There were no priorities to be determined because the manner of presentation was fixed.
One reason for this manner of presentation is that all of the data, including graphics and text, would pass through a single frame buffer so it would simply provide the information on the screen in accordance with the data inputs. Often in such cases there would be a central processing unit (CPU) for graphics and a separate CPU for text but the outputs of both of these CPU's would be connected to the single frame buffer. In order for this frame buffer to handle the data, the CPU's would have start and stop. The frame buffer would tend to be somewhat overloaded and the speed of operation would be reduced. In some instances where it was determined to have only text where there had been both, the graphics would be deleted but the rest of the screen would be blank rather than have the text fill in the unused space.